Break-induced replication: what is it and what is it for?

Homologous recombination (HR) is considered to be an error-free mechanism for the repair of DNA double-strand breaks (DSBs). Indeed, most DSB repair events occur by a non-crossover mechanism limiting loss of heterozygosity (LOH) for markers downstream of the site of repair and preventing chromosome rearrangements. However, DSBs that arise by replication fork collapse or by erosion of uncapped telomeres have only one free end and are thought to repair by strand invasion into a homologous duplex DNA followed by replication to the chromosome end (break-induced replication, BIR). As BIR from one of the two ends of a DSB would result in a long tract of LOH it suggests BIR is suppressed when DSBs have two ends in order for repair to occur by a more conservative HR mechanism. Recent studies showed that BIR can occur by several rounds of strand invasion, DNA synthesis and dissociation resulting in chromosome rearrangements when dissociation and reinvasion occur within dispersed repeated sequences. Thus template switching BIR can be highly mutagenic and this process could be important for genome evolution and disease development.